Along with the smaller size and increased thinness of electronic components, electronic equipment, etc. in recent years, smaller size and increased thinness of the circuit boards etc. which are used for these have been sought. For this reason, along with this, in the circuit boards, it is required to form the higher density circuit wiring patterns.
To form such high density circuit wiring patterns, multilayer structure circuit boards have been used, and the layers which form the circuit boards have been made smaller in thicknesses. When increasing the thinness of such a multilayer structure circuit board, from the viewpoint of enabling the mechanical strength of the circuit board to be maintained even when increasing the thinness, the method of using a prepreg which contains glass fiber or other fiber base material as the material which forms the interlayer insulating layer of the multilayer structure circuit board has been studied.
For example, Patent Document 1 discloses the art of using a resin varnish which contains an epoxy resin to obtain two types of thermosetting resin films with different thicknesses and sandwiching glass fibers between the obtained thermosetting resin films to obtain a prepreg. However, in the art of this Patent Document 1, the cured shaped article which is obtained by curing the prepreg is large in variation of surface roughness. For this reason, when using electroless plating etc. to form a conductor layer, there was the problem that the metal plating film was low in formability and the conductor layer could not be formed well. Further, in the art of this Patent Document 1, if reducing the thickness of the obtained prepreg to deal with the increased thinness of multilayer structure circuit boards, there was the inconvenience that when laminating the prepreg on the circuit patterns, the glass fibers which are contained in the prepreg are pushed up by the circuit patterns, the glass fibers end up being exposed at the prepreg surface due to this, and therefore the cured shaped article which is obtained by curing the prepreg ends up varying more greatly in surface roughness.